Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe Spectroscopy

The photochemistry of the cationic diazidocobalt(III) complex, trans-[Co(cyclam)(N-3)(2)](+), following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR), thereby enabling a combined probing of electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state, (MC)-M-1(E-1(g)), into the triplet ground state, (MC)-M-3 (E-3(g)/(3)A(Ig)) on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from monoazidocobalt(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt(III) species. At the same time, the (MC)-M-3 state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT state. The (LMCT)-L-1 lifetime is about 2 ps, whereas that of the excited state, (MC)-M-1, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from (MC)-M-3 to the parent's singlet ground state, (1)A(Ig), occurs with a rate of 1/(110 ps). The mid-infrared pump-probe spectrum after 1 ns, gives evidence for a heterolytic Co-N bond fission with a quantum yield of similar to 5%, leading to free azide anions and the monoazido species, trans-[Co(cyclam)(N-3)(OSMe2)](+), featuring an oxygen-bound DMSO ligand in its coordination sphere.